The BT-H have this rev-counter information on their webpage:-
"Question 16: Can I use an electronic rev counter with my new BTH magneto?
Answer: Several people are successfully using the self-powered the Scitsu electronic rev counter. Nick Brown, of A.J.S. Motorcycles, has sent us this advice. (See Note 2.) Another rev counter we have successfully tried is sold by Mick Hemmings. This simply needs a diode (IN4001 or similar) in the sensing lead and a12 volt supply.
Re: Scitsu Tachometers: The single Magnetos use a yellow unit. The black model is recommended for a 360 twin with BT-H high output mag. You should connect the Tacho sensor wire to both HT leads. V twins are tricky because of the uneven firing pulse. the 1-2 space, 1-2 space, firing order can confuse the Tacho, so it is best to treat V twins like a single and try to keep the sensor wire away."
Mick Hemmings died a while back and I think his rev-counter supplier had stopped making them. These used a 12 volt supply for the electronics and the sensing wire was wrapped around the plug lead.
The Scitsu units worked well and had their own rechargeable battery. They maybe also have made one that could be powered. The sensing wire wrapped around a plug lead. Don't try to feed any rev-counter direct from the output from leads to the spade connectors on the coil/s as the output to the coils in the hundreds of Volts.